This application claims priority under 35 U.S.C. xc2xa7xc2xa7119 and/or 365 to 01106195.9 filed in Hong Kong on Sep. 3, 2001; the entire content of which is hereby incorporated by reference.
The present invention relates to battery chargers for charging a plurality of rechargeable batteries connected in series. More particularly, this invention relates to battery chargers having a plurality of serially connected battery charging sections. More specifically, although not solely limiting thereto, this invention relates to serial battery chargers in which a battery in any one of the serially connected charging sections can be removed or bypassed without materially affecting the charging conditions of the batteries remaining in other charging sections of the serial battery charger. Furthermore, this invention relates to serial battery chargers in which there is utilized a simple electronic element which provides a low-impedance to the charging circuit during charging, a high-impedance to block reverse current flow from a battery when there is no power supply to the charging section and a comparatively high-impedance when the charging section is shunted or by-passed.
Re-chargeable batteries are widely used in a lot of portable or mobile electrical and electronic devices or appliances such as, cellular or cordless telephones, remote repeaters, remote control units, remote sensors, portable lighting devices, portable radios, portable drills and many other devices. Re-chargeable batteries are generally preferred over disposable batteries nowadays because they are more environmental friendly and provide longer term cost savings. For remote applications, rechargeable batteries are probably the only practical choice.
Re-chargeable batteries require repeated charging in order to supply electrical power to the devices or appliances in which they are installed. Nowadays, portable devices usually require a plurality of batteries to operate and the batteries required are typically in the range of two to ten batteries. Hence, it is desirable that there can be provided intelligent battery chargers which can charge a plurality of re-chargeable batteries at the same time. There are two main types of battery chargers. The first type is the parallel charger in which all the batteries are subject to the same charging voltage but are charged with different charging currents. The other type is the serial charger in which the batteries being charged are connected in series and the same charging current usually passes through all the serially connected batteries.
In applications in which batteries are alternatively charged and discharged, a power supply of 3 to 12 volts is generally required while the voltage of each rechargeable battery is typically in the region of 1-2 volts. In those applications, batteries are typically connected in series for charging and discharging. For charging batteries for use in such applications, a serial battery charger must be used.
Because of the wide-spread use of rechargeable batteries, there are increasing demands for fast battery chargers which are capable of fully charging an empty battery in about an hour (the xe2x80x9c1Cxe2x80x9d chargers) so that users do not have to wait for too long before the batteries are sufficiently charged for use. For example, for a 1,600 mAH re-chargeable battery, the 1C current rate is about 1.6A. In order to facilitate fast and efficient battery charging, battery chargers generally utilise high frequency pulsed charging current having a relatively high current rate. When a battery is being charged, it will produce oxygen on the electrode and the consumption of oxygen by the negative electrode will cause the battery to heat up. In general, charging at the current rate of 1C is preferred because this charging rate is regarded as striking a balance between reducing charging time and maintaining a healthy battery under current battery technologies. Of course, with further advance in battery technologies, batteries may be charged at even higher current ratings without over-heating. If that happens, battery chargers supplying higher charging rating than 1C will be expected to become more popular. In general, fast battery chargers, especially those for charging small voltage re-chargeable batteries of about 1.5-2V, are preferably configured so that the batteries are charged in series. This is because if the batteries are fast charged in parallel, a power supply having a very large current supply rating will be required and this may be very costly.
On the other hand, a serial connection implies that the same current must flow through each serially connected charging section. This may also create great difficulty in a lot of circumstances. For example, when a battery is removed from the charger upon completion of charging to avoid overheating or damaging or because it is already defective, charging will be disrupted until a replacement battery has been inserted into the charger. Similar problems also arise if rechargeable batteries of different capacities are charged together or good batteries are mixed with bad ones. This is because when a battery of a smaller capacity has been fully charged, there is a good chance that a battery of a larger capacity still requires charging. For simple serial chargers with no monitoring and control circuits, the batteries will be continuously charged. As a result, overheating, battery damage or even explosion may result. On the other hand, for those more sophisticated serial battery chargers with charging conditions monitoring and charge control circuits, the battery charger may shut down once any one of the batteries being charged is detected as being fully charged. This is obviously undesirable as the remaining batteries may still require further charging. Furthermore, whenever batteries are inserted or removed from a serial battery charger during the charging process, the whole charging process will be interrupted. Hence, it is desirable if there can be provided intelligent serial battery chargers which allow serial charging of re-chargeable batteries in which the charging currents supplied to the individual batteries in serial connection are largely independent of that supplied to other batteries.
For many battery chargers, it is known that, when power supply to the battery charger is turned off, there may be a reverse leakage current which flows from the battery to the charger or the peripheral circuitry. Reverse leakage current among the serially connected batteries could also cause reverse charging of individual batteries by other batteries that are connected in the series charger. This is clearly undesirable which may cause draining of the full battery capacity and may even damage the charger. Hence, it is desirable that each charging section of a serial battery charger is provided with means to prevent undesirable reverse current leakage as well as a by-passing circuitry so that the charging conditions of one individual charging section would not affect the charging conditions of the other charging sections.
Many by-passing circuits, circuit arrangements or topologies have been proposed to alleviate the adverse influence of the charging conditions in a serial charging section to other charging sections. While serial chargers having arrangements to by-pass some or all of the charging sections have been known, they are generally very complicated and do not simultaneously include means or circuits to prevent reverse leakage or discharge from the batteries.
To provide a serial battery charger which fulfils the above requirements is a difficult task because several conflicting requirements need to be met. Firstly, in order to prevent reverse current leakage or adverse current discharge from the battery, a blocking device which has a high reverse impedance must be inserted in series with the battery. Secondly, that serial block device must have a low impedance when there is a forward current which flows into the battery for battery charging. On the other hand, if the blocking device has a low forward impedance when the by-passing switch has been activated (which usually occurs when there is still power supply to the battery charging terminals), that low-impedance blocking device will compete with the by-passing switch for the supplied current and, as a result, adverse charging current will keep flowing into the batteries. In addition, that blocking device must have a high impedance when the by-passing switch has been activated, otherwise, a large and un-desirable current will flow in a current loop which is formed by the battery, the blocking device and the by-passing switch. Hence, it is highly desirable if a serial battery charger which can fulfil the above conflicting requirements can be provided. It will be even more desirable if such improved battery chargers can be realised using simple circuit blocks and components so that high reliability as well as low costs can be achieved.
It is therefore an object of the present invention to obviate the problems or shortcomings associated with existing or known serial battery chargers. In particular, it is an object of the present invention to provide a circuit arrangement for an improved battery charging section which can be used in serial chargers so that the charging section can be shunted or by-passed when selected and, at the same time, providing blocking means to prevent reverse current.
An important objective of the present invention is therefore to provide an intelligent serial battery chargers in which the charging current or charging conditions of one battery in the serial connection is largely unaffected by the charging conditions of other batteries in the serial connection.
An equally important object of the present invention is to provide a serial battery charger in which a battery can be removed from the serially connected battery at any time without disrupting the charging of other batteries and, at the same time, adverse reverse current flow from a battery can be avoided.
As a minimum, it is an object of the present invention to provide the public with a choice of serial battery chargers which are provided means to obviate undesirable battery discharge when the battery charger is not supplying charging power and to provide useful battery by-pass when necessary.
According to the present invention, there is provided an improved serially connected battery charger which includes at least first and second parallelly connected branches, said first parallel branch includes an electronically controllable by-passing switch and said second parallel branch includes positive and negative terminals for receiving respectfully the positive and negative terminals of a battery and an one-way electronic device connected in series, said by-passing switch has a very low impedance when turned-on and a very high impedance when turned-off, said one-way electronic device is characterised in that it has a very low-impedance when current flows from said charging section into said battery terminals and it has a high-impedance when said by-passing switch is turned on.
According to another aspect of the present invention, there is provided a charging block for use in a serial battery charger including at least first and second parallelly connected branches, said first parallel branch includes an electronically controllable by-passing switch and said second parallel branch includes positive and negative terminals for receiving respectfully the positive and negative terminals of a battery and an one-way electronic device connected in series, said by-passing switch has a very low impedance when turned-on and a very high impedance when turned-off, said one-way electronic device is characterised in that it has a very low-impedance when current flows from said charging section into said battery terminals and a high-impedance when said by-passing switch is turned on.
According to a third aspect of the present invention, there is provided a serial battery charger including a battery charging section which includes at least first and second parallely connected branches, wherein said first branch includes a diode serially connected with the terminals for connecting the battery to be charged and said second branch includes a MOSFET by-passing switch, said by-passing switch is connected across said first branch and provides low-impedance shunting when activated, said blocking diode has a low-impedance when current flows into said battery to be charged and has a high-impedance when there is no power supply from said battery charger or when said by-passing switch is turned on.
According to a fourth aspect of the present invention, there is provided a battery charger including a plurality of battery charging sections which are connected in series, wherein each said charging section includes a first and a second parallelly connected branches, said first parallel branch includes an electronically controllable by-passing switch and said second parallel branch includes positive and negative terminals for receiving respectfully the positive and negative terminals of a battery and an one-way electronic device connected in series, said by-passing switch has a very low impedance when turned-on and a very high impedance when turned-off, said one-way electronic device is characterised in that it has a very low-impedance when current flows from said charging section into said battery terminals and it has a high-impedance when said by-passing switch is turned on.
Preferably, the battery charger further including a micro-controller, the micro-controller monitors a set of parameters of the battery being charged and activates said by-passing switch by forming a low-impedance shunting across said first parallel branch when some or all of said measured battery parameters satisfies a set of pre-determined conditions.
Preferably, the one-way electronic device is a diode.
Preferably, the by-passing switch is a field-effect-transistor (xe2x80x9cFETxe2x80x9d), including a MOSFET.
Preferably, the gate of said by-passing MOSFET is connected to a microcontroller which controls the gate voltage of said MOSFET to turn on or turn off said MOSFET such that when said MOSFET is turned on, the impedance across the drain-source terminals of said MOSFET is low, thereby activating the by-passing function, and, when said MOSFET is turned off, the impedance across the drain-source terminals is very high, thereby de-activating the by-passing function.